Katsuobushi (Japanese: 鰹節) is simmered, smoked and fermented skipjack tuna (Katsuwonus pelamis, sometimes referred to as bonito). It is also known as bonito flakes. Katsuobushi or similarly prepared fish is also known as okaka (おかか).

Shaved katsuobushi and dried kelp—kombu—are the main ingredients of dashi, a broth that forms the basis of many soups (such as miso) and sauces (e.g., soba no tsukejiru) in Japanese cuisine.

Katsuobushi's distinct umami taste comes from its high inosinic acid content. Traditionally made katsuobushi, known as karebushi, is deliberately fermented with Aspergillus glaucus fungus in order to reduce moisture. Katsuobushi has also been shown to impart "kokumi" (i.e. enhances flavor).

Traditionally, chunks of katsuobushi were shaved as needed with an instrument similar to a wood plane called a katsuobushi kezuriki.

Today katsuobushi is typically sold in bags of small pink-brown shavings, which vary by thickness: smaller, thinner shavings, called hanakatsuo (花鰹), are used as a flavoring and topping for many Japanese dishes, such as okonomiyaki, while the larger thicker, called kezurikatsuo (削り鰹), are favored for making the widely used dashi stock.

In addition to making dashi, other popular uses of katsuobushi include:

Okaka, finely chopped katsuobushi dressed with soy sauce.
As a stuffing for rice balls (onigiri).
As a topping for rice. Popular for bentō, often covered with strips of laver.
Dried okaka is used as an ingredient of furikake rice topping (called "okaka furikake").
As a seasoning for cold tofu (hiyayakko, 冷奴) along with grated ginger and Welsh onion (a type of spring onion).
Sprinkled with sesame seeds and chopped laver atop cold soba noodles (zarusoba).
As a topping on takoyaki and okonomiyaki.
As a seasoning on century egg along with sesame oil and soy sauce.
As a high-protein treat for cats sold at pet stores.
As a topping for ramen mixed with salt.